gitlab-ci: Run merge request pipelines automatically only for Marge Bot

MR pipelines not triggered by Marge Bot can still be triggered manually.

Motivation: The main & forked Mesa project CI pipelines combined are
currently generating over 1 TB of egress traffic per week. ~80% of this
is from pre-merge pipelines. Assuming this corresponds to 4 pre-merge
and one post-merge pipeline per MR on average, this change could
potentially eliminate up to ~60% of the overall traffic (by preventing
3 of the 4 pre-merge pipelines from running automatically).

(Of course, this could be subverted if all jobs of the other pipelines
were triggered manually anyway... In most cases, manually triggering
just a few jobs should suffice)

v2:
* $GITLAB_USER_NAME was the wrong variable, $GITLAB_USER_LOGIN should
  do the trick.

Suggested-by: Marek Olšák <[email protected]>
Reviewed-by: Adam Jackson <[email protected]>
Acked-by: Alyssa Rosenzweig <[email protected]>
Reviewed-by: Pierre-Eric Pelloux-Prayer <[email protected]>
Reviewed-by: Samuel Pitoiset <[email protected]>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4432>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4432>

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